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What is the difference between fill weight and fill power?

Quantity of fill
The more down you have in a bag the warmer it is, this is known as the Fill weight, (don’t confuse this with “Fill power” which is a measure of how good the down is: not all down is the same) This means that you can have a sleeping bag which either has:
Low fill power and Higher Fill weight (cheaper but bulkier and heavier)
Or
Low fill weight and high Fill power (more expensive but more compact and lighter)

Fill Power
There is no substitute for good quality down and the easiest way to measure this is to check out its Fill Power, the higher the number the better it is. But attention, just because some thing has an extra hundred here and there doesn’t mean it substantially better.

The numbers game
In the last few years several brands have developed 900 fill power. Be aware, evolution doesn’t happen so fast, they haven’t developed Superduck. What they have done is get a bit better at sorting the pure down from the feathers and a awful lot better at getting the best test results from the same down quality they used before.

For what to look for when choosing a down sleeping bag, have a read of our handy guide.

Updated: Oct 21, 2016